Question

which best explains whether a triangle with side lengths 2in , 5in , 4in. is an acute triangle

Answer 1

so we know that 3,4 and 5 are pythagorean triplet and these result from a right triangle sides length 
- than 3,4 and 5 mean right triangle so 2,5 and 4 mean acute triangle sure or you can prove it in this way too
- than 3^2 +4^2 =5^2 => a right triangle so than 
2^2 +4^2 < 5^2 => an acute triangle 

hope helped